1. Field of the Invention
The present invention relates to methods of forming a resist pattern with a photoresist on a substrate, of forming a metal electrode pattern, and of manufacturing a surface acoustic wave device.
2. Description of the Related Art
The following method is known as a method of forming a metallic resist pattern. FIGS. 1A to 1C show a known method of forming a resist pattern.
In this method, a substrate 1 is coated with a positive or negative photoresist 3 (FIG. 1A), and the photoresist 3 is exposed to an ultraviolet light with a wavelength of 365 nm or less through a photomask 10 provided with holes to form a desired pattern (FIG. 1B). Then, a resist pattern 4 is formed by developing the photoresist 3 so as to partly expose the substrate 1 (FIG. 1C).
However, when the substrate is formed from a transparent material such as LiTaO3, the resist-pattern-forming areas of the photoresist, which are normally not exposed to the light because of the protection by the photomask, are exposed to diffusely reflected light caused by the transparent substrate. Then, part of the resist pattern which is normally not developed is developed and removed, and consequently, the desired resist pattern cannot be formed. Furthermore, when a finer resist pattern having a reduced width is formed, part thereof may be removed, and at worst pattern deformation may occur.
In order to solve this problem, an antireflection film formed of an organic polymer containing a colorant capable of absorbing ultraviolet light with a wavelength of about 365 nm has been provided at the back of the substrate to prevent the diffusely reflected light. Alternatively, an antireflection film formed by laminating a silicon layer and a lightproof metallic layer in that order has been provided at the back of the substrate.
These antireflection films, however, have caused the following problems. First, using the organic polymer for an antireflection film causes contamination of the stage of the stepper, thereby making accurate light exposure difficult. In other words, the accumulated residue of the organic polymer causes a degraded flatness of the stage, thereby deteriorating the accuracy of light exposure.
Second, the photoresist is cured at about 100xc2x0 C., whereas the organic-polymer antireflection film is cured at a high baking temperature of about 200xc2x0 C. As a result, if the transparent substrate is pyroelectric, it is likely to be damaged by electrostatic charge. In order to solve this problem, an additional conductive film may be provided on the surface of the organic-polymer antireflection film to discharge the static electricity caused by the temperature variation. Unfortunately, providing the conductive film on the surface of the antireflection film increases the number of the processes, inevitably increasing the cost.
Third, when the antireflection film formed by laminating a silicon layer and a lightproof metallic layer in that order is used, the favorable range of the film thickness is limited, and an additional process, which increases the cost, is required to form the lightproof metallic layer. In addition, since the refractive index of silicon for ultraviolet rays is 0.57 to 0.77, the silicon layer cannot adequately inhibit the reflection of ultraviolet rays when the material of the substrate has a refractive index of 1.5 or more.
Accordingly, it is an object of the present invention to provide a method of forming a resist pattern in which an antireflection film capable of preventing the diffuse reflection caused by the transparent substrate is formed.
To this end, according to one aspect of the present invention, there is provided a method of forming a resist pattern. The method comprises the steps of forming a substrate with a material having a refractive index n1 and forming an antireflection film on the rear surface of the substrate with a semiconductor having a band gap energy of 3.4 eV or less and a refractive index n2. In this instance, the reflectance expressed by (n1xe2x88x92n2)2/(n1+n2)2 is 0.15 or less. The method comprises the steps of coating the substrate with a photoresist and radiating an ultraviolet light on the photoresist downward through a photomask so that the photoresist becomes partly soluble in a developer. Also, the method comprises a step of soaking the substrate in the developer to remove the soluble photoresist.
The substrate may be formed of LiTaO3 and the antireflection film may be formed of a material having a refractive index n1 in the range of 1.2 to 4.6.
Alternatively, the substrate may be formed of LiNbO3 and the antireflection film may be formed of a material having a refractive index n1 in the range of 1.0 to 4.0.
The antireflection film may be formed of a material selected from the group consisting of ZnO, ZnSe, ZnTe, CdSe, CdS, CdTe, and TiOx (0 less than xxe2x89xa62).
The method may further comprise a step of mirror-finishing the rear surface of the substrate.
The mirror-finished surface of the substrate may have a roughness of 50 nm or less.
The antireflection film may be formed of ZnO or ZnO containing a metal.
The metal may be Ni.
The developer may be alkaline.
The antireflection film may have a thickness in the range of 0.05 to 2 xcexcm.
The resist-pattern-forming method includes forming the inorganic semiconductive antireflection film of ZnO, ZnSe, ZnTe, CdSe, CdS, CdTe, TiOx (0 less than xxe2x89xa62), or the like on the rear of the substrate, having a band gap energy of 3.4 eV or less, such that the reflectance expressed by (n1xe2x88x92n2)2/(n1+n2)2 is 0.15 or less, wherein n1 and n2 are the refractive indexes of the substrate and the antireflection film, respectively. This method does not cause contamination of the stage of the stepper nor does it require any additional antireflection layers. By applying this method, the diffuse reflection of ultraviolet light and pattern deformation are prevented so that reliable fine resist patterns can be manufactured.
According to another aspect of the present invention, a method of forming an electrode pattern is provided. The method comprises the steps of forming a resist pattern by the resist-pattern-forming method of the present invention and forming a metal film on the partly exposed surface of the substrate and the resist pattern. Also, a step of removing the resist pattern together with the metal film formed thereon is provided.
The electrode-pattern-forming method does not lead to a partly removed resist pattern by the developer nor does it cause pattern deformation even when the resist pattern is finely formed to have thin line widths. The method therefore achieves reliable fine electrode patterns.
According to another aspect of the present invention, a method of manufacturing a surface acoustic wave device is provided. The method comprises the above method of forming a resist pattern or the above method of forming an electrode pattern. The methods of forming a resist pattern and an electrode pattern lead to fine resist patterns without problems such as pattern deformation, and the surface acoustic wave device manufactured by either of the methods can have a reliable fine IDT (interdigital transducer). In particular, the methods contribute to an improvement in the performance of the surface acoustic wave device requiring a very fine pattern for the use in the frequency band of 1 GHz or more and to increasing the process yield.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.